The present invention relates to a read-only memory (ROM) device and, more particularly, to a read-only memory device in which the data "0" or "1" corresponds to the presence or absence of a metal insulator semiconductor MIS transistor in a memory cell.
In a read-only memory device, which is a kind of random-access memory device, programming of the data pattern is fixed during manufacture and in addition, the data pattern can not be changed during operation. Therefore, the data pattern must be determined prior to the manufacture of such a read-only memory device.
According to one conventional read-only device of a MOS (broadly, MIS) transistor type, a gate mask is prepared for the above-mentioned data pattern so as to form enhancement type MOS transistors alternatively within memory cells located at intersections of word lines and bit lines. For example, in a memory cell, the data "0" corresponds to the absence of a MOS transistor, while the data "1" corresponds to the presence of a MOS transistor. In more detail, a gate oxide layer at the intersection corresponding to the data "0" is relatively thick so as to increase the threshold voltage thereof, which means that no transistor is substantially formed. Contrary to the above, a gate oxide layer at an intersection corresponding to the data "1" is relatively thin so as to decrease the threshold voltage thereof, which means that a MOS transistor is formed.
However, in the above-mentioned conventional device, such a phenomenon that a large number of MOS transistors are connected to one word line occurs. For example, when all the data belonging to one word line is "1", the number of MOS transistors connected to the word line reaches the maximum, which increases the capacitance of the word line due to the presence of a large number of thin gate oxide layers. As a result, the rising or falling of the potential of the word line corresponding to a change from the selected mode to the non-selected mode or vice versa becomes the slowest. That is, the access time for such a word line becomes the longest.